The number of source packages in each Debian release (stable, testing, unstable
and experimental) is illustrated below. Note that removals from experimental
do not necessarily happen automatically when the package in experimental
is superseded by a package in unstable.

Source packages may be updated with a new Debian revision (1.2.3-1 → 1.2.3-2)
or a new upstream version (1.2.3 → 1.2.4).
The number of source packages that have an updated package between releases
is show below. Note that:

immediately following release, testing is identical to stable

packages migrate from unstable into testing (quite rapidly following
the release) thus increasing the differences between stable and testing

once testing migration is frozen to prepare for a release, testing
and unstable start to diverge more and more

during the freeze, experimental is used both as a test-bed and also
to house new versions that would not make it into the next release but
whose upload to unstable could be disruptive to the release preparations.

New upstream versions rather than just new Debian revisions tend to be of
more interest to end users. New upstream revisions tend not to be uploaded
to unstable during a freeze as there is essentially no chance of them being
included in the upcoming release but having them present means that packages
that pick them up as dependencies cannot migrate to testing to fix bugs.
Note that packages adding an epoch (1:) to their version or repackaging the
upstream source and adding a marker like +dfsg to their version (perhaps to
remove non-free material like RFCs or sourceless binaries) will also look like
"new upstream" versions in this metric; however, there aren't that many such
changes.

Over time, new upstream packages are added Debian. In the Debian context, new
packages are ones that a source package name that does not exist in the archive
already. New packages can be:

simply a new upstream version of the package but with a different
source package name; this is only for a very few Debian packages where
multiple upstream versions are supported at the same time
(e.g. gcc-4.4 and gcc-4.7, python2.6 and python2.7)